As an alternative to the use of cast-on block pieces, which as a rule are made of metal, WO 2007/017385 A2 and DE 10 2005 038 063 A1 teach the use of block pieces made of plastic which would provide sufficient support to the spectacle lens blank. This, however, involves two major drawbacks. On the one hand, the adhesive materials which are currently available to affix the spectacle lens blank to the plastic block piece lack the required holding power. To achieve sufficient holding power, the spectacle lens blank and the block piece must be pressed against each other, thus possibly deforming the spectacle lens blank in an adverse manner, which adversely affects the geometry of the glass or lens, at least once it has been removed from the block. On the other hand, the plastic block pieces have a very low strength, thus adversely affecting the holding position within a chuck of a machine tool.
DE 103 04 388 A1 discloses a block piece for spectacle lens blanks which comprises a coupling part and a holding part. The holding part can be adapted in shape so as to conform and adhere to the spectacle lens blank. The necessary holding power is achieved by heating and pressing, adding an adhesive material if required. The deformation of the spectacle lens blank caused by said pressing is a disadvantage since it adversely affects the geometry of the spectacle lens or lens, at least once it has been removed from the block.
A method for connecting a block piece to a spectacle lens blank by means of a connecting material is already known from DE 696 04 116 T2. The connecting material consists of a heavy metal alloy in this case, which must by no means be machined or damaged during the subsequent process of machining the spectacle lens blank. Therefore, the spectacle lens blank is only supported by the connecting material in the area of the diameter which will in no case be machined, taking into account a safety margin. The diameter cannot be reduced further until the block piece, including the connecting material, has been removed.